One known mechanism for shifting the drive wheels to which engine power is transmitted from front or rear wheel drive to all four wheel drive is a transfer case. Generally speaking, the transfer case is fabricated integrally with the transmission. The transfer case is specifically adapted for use in connection with a particular transmission and the use of a different transfer case requires the use of or replacement of many different parts. Thus, the transfer case is not well suited for use with different vehicles and different transmissions.
Another known arrangement for effecting a shift between two-wheel drive and four-wheel drive is disclosed in Japanese Patent Publication No. 23330/1989. Here, a clutch is mounted to one output shaft of the differential on the axle side for coupling and uncoupling the engine power from a part-time four-wheel drive vehicle. When the engine power is coupled, the clutch is engaged to obtain the four-wheel-drive mode. When the engine power is uncoupled, the clutch is disengaged to achieve the two-wheel-drive mode. In this way, the drive mode can be switched between the two-wheel-drive mode and the four-wheel-drive mode. Also, in the two-wheel-drive mode, the parts of the drive mechanism necessary for the four-wheel-drive mode are not needlessly driven.
In the arrangement disclosed in the above-cited Japanese Patent Publication No. 23330/1989, the clutch assembly is mounted on the output shaft side of the differential which undesirably increases the width of the differential. This clutch assembly also transmits torque after the fmal gear reduction. In the four-wheel-drive mode, large torque is imposed on the clutch assembly. Therefore, there arises the problem that the clutch assembly must be made of a material having great strength or must be made rather large in size to obtain sufficient strength.
In light of the foregoing, a need exists for a drive-mode shifting mechanism that does not suffer from the foregoing disadvantages and drawbacks.
It would thus be desirable to provide a drive-mode shifting mechanism for switching between two-wheel drive and four-wheel drive that need not be made of excessively strong material and that is not excessively large in size.
It would also be desirable to provide a drive-mode shifting mechanism for switching between two-wheel drive and four-wheel drive that has a wide range of use and is not limited to use in connection with a particular vehicle or transmission.